The present invention relates to a device for operating a discharge lamp by means of a high frequency inverter with a dimming function, and more particularly to a discharge lamp operating device which has an excellent start characteristic and is capable of operating the discharge lamp with a sufficiently long life.
When a liquid crystal display panel is used as a display panel of an automobile, the light source thereof is required to have a sufficient luminance and low power consumption. In order to keep an optimum luminance to prevent fatigue of a driver when he or she drives a car in a bright daytime and a night time, the luminance of the light source must be switched in at least two steps, for example, 50% and 100%. Bearing in mind the fact that the optimum luminance varies from driver to driver, it is desirable that the illumination can be finely variable or dimmed between 0% and 50%.
A discharge lamp such as a fluorescent lamp can provide a sufficient luminance with a low power consumption.
In a common method for dimming the discharge lamp such as fluorescent lamp, a single fluorescent lamp is used, and a switch is inserted in series with the primary winding of an oscillation transformer in an operating device and the switch is turned on and off to change the duty factor to increase or decrease the discharge current so that the luminance is controlled. The fluorescent lamp uses an electron emission electrode which has to be heated. The electrode is usually heated by self-heating by the discharge current.
The above method has two problems 1. In a dimming mode, the discharge current decreases and the self-heating decreases and the electrode temperature drops. As a result, electrode spattering by ion impaction increases and the lifetime of the fluorescent lamp is shortened. 2. If the lamp is started or turned on in the dimming mode, the voltage applied to the lamp is zero when the input switch is off. Since the starting of the lamp depends on the applied voltage and the duration thereof, the lamp start characteristic in the dimming mode is lowered, that is, the lamp is hard to start.
In order to resolve the above problem, in Japanese Unexamined Patent Publication No. 57-162295, a preheating circuit is provided and the electrode is heated by only the discharge current when the discharge current is at a rated value, and in the dimming mode, a small electrode heating current is supplied by the preheating circuit during a turn-off period, that is, during an off-period of the input power supply. In this method, the electrode heating current in the dimming mode cannot be increased to a desired level and it is not effective to improve the start characteristic or prevent blackening and shortening of the lifetime of the lamp. Further, the preheating circuit is complex and expensive.